The field of the invention is that of integrated circuits having on-chip power supplies.
On-chip voltage regulators and dc to dc converters are increasingly used in integrated semiconductor chips. Charge pumps are typically used to convert supply voltages to higher voltages or to lower voltage. In voltage converters, the standard supply voltage is used to drive an oscillator. The oscillator signal is used in turn to charge the output up or down to the required value. Charge pumps usually use voltage regulation to compensate for process and supply voltage variations and to maintain the output at the required voltage level. They also use large decoupling capacitors to reduce ripple voltage when load current is drawn from the regulated output.
When the magnitude of the converted voltage output goes below the required levels, one or more pump cycles are needed to restore the output back to the required voltage.
The oscillator frequencies used in these charge pumps are typically very small compared to the frequency of the active cycle. For example, a memory chip with-access time and cycle time less than 10 ns may use oscillator frequencies 1 MHz to 20 MHz. Even during an active cycle, load current is drawn from the regulated output only during a fraction of the active cycle. For example, for a system with active clock period of 10 ns, load current may be active only for 2 ns. The lower oscillator frequencies are used in the charge pump to minimize inefficiencies of the charge pump, as well as to reduce power consumption. As a result, several active chip cycles may take place during one pump cycle. A large decoupling capacitor is necessary during this time, to provide charge for the load current with low ripple in the output voltage of the charge pump. Decoupling capacitors occupy considerable chip area. Planar gate area capacitors, or trench capacitors, may be used for decoupling. Trench decoupling capacitors would use less area compared to planar capacitors, but trench capacitors would add to processing cost. Trench capacitors also have higher ohmic and parasitic losses associated with it.
The invention relates to a circuit technique that not only eliminates the need for large decoupling capacitors, but restores the output voltage to the required level at a faster rate.
A feature of the invention is an on-chip power supply that employs at least two decoupling capacitors connected in parallel.
A feature of the invention is the use of a smaller decoupling capacitor together with a supplementary capacitor for supplying reserve charge.
Another feature of the invention is a controllable connection for connecting the two capacitors in parallel when the output node declines in voltage by a threshold amount.
Another feature of the invention is the maintenance of the supplementary capacitor at a higher voltage than the decoupling capacitor.